Although solid oxide fuel cells, so-called, the third generation fuel cells, were studied later than phosphoric acid fuel cells (PAFCs) and molten carbonate fuel cells (MCFCs), the solid oxide fuel cells are expected to be in practical use in the near future, subsequent to the PAFCs and the MCFCs, by virtue of the recent rapid development of material technologies. To commercialize the solid oxide fuel cells, advanced nations have devoted tremendous effort to the fundamental research on SOFC and enlarged production scale.
Such a solid oxide fuel cell is operated at a high temperature ranging from about 600° C. to about 1,000° C., and has advantages in that it is the most highly efficient of existing fuel cells, there are few pollutants discharged, a fuel reformer is not necessary, and a combined power generation is feasible.
The solid oxide fuel cell may be largely classified into a tubular solid oxide fuel cell and a planar solid oxide fuel cell. According to the planar solid oxide fuel cell, a planar solid oxide fuel cell stack has a power density greater than that of a tubular solid oxide fuel cell stack. However, it is difficult to manufacture a large-area fuel cell due to a gas sealing problem, a thermal shock generated by a thermal equilibrium coefficient difference between materials, and the like. In recent years, studies with respect to the tubular solid oxide fuel cell are being actively carried out. The tubular solid oxide fuel cell may be classified into an air electrode-supported type and a fuel electrode-supported type. However, in the case of the air electrode-supported type, the air electrode-supported type suffers from a disadvantage of a relatively high manufacturing cost because air electrode materials, such as lanthanum (La), manganese (Mn), etc., are very expensive, and LaSrMnO3 (LSM) that is a raw material for the air electrode is difficult in manufacturing. In addition, the unit cell has a low mechanical strength and does not withstand impact because the air electrode serving as the support is made of ceramic. To solve the above-described problems, a fuel electrode-supported solid oxide fuel cell using a fuel electrode as the support has been developed.
The tubular fuel electrode support used in the fuel electrode-supported solid oxide fuel cell satisfies demands required as both the electrode and the support, and is advantageous in that co-sintering is feasible because reactivity between the support and an electrolyte layer is low, and a stable fuel cell stack is fabricated due to a high mechanical strength of the support. To generating high power by using the tubular solid oxide fuel cell, a stack of the tubular solid oxide fuel cell has to be provided. Also, to manufacture the stack of the tubular solid oxide fuel cell, it is necessary to develop a connector for electrically connecting unit cells to each other.